The use of optically transmitted signals in communication systems is dramatically increasing the throughput rate of data transfer. In a typical network configuration, an electrical signal is converted into an optical signal by either a laser diode or a light emitting diode (LED). The optical signal is then transported through a waveguide, such as an optical fiber, to an optical detector that converts the optical signal into an electronic signal.
A unit can be assembled that incorporates components for performing many of these functionalities in a single module. Such a module may comprise an integrated circuit (IC), one or more light sources such as an LED or a laser diode, and one or more optical detectors such as silicon, InP, InGaAs, Ge, or GaAs photodiode. The optical detector is used to detect optical signals and transform them into electrical waveforms that can be processed by integrated circuitry in the IC. In response, optical signals are output by the light sources, which may be controlled by the circuitry in the IC. The optical detector(s) may be formed, for example, on a silicon, InP, InGaAs, Ge, or GaAs substrate while the optical source(s) are included, for example, on a GaAs, InGaAs, InP, InGaAsP, AlGaAs, or AlGaAsSb substrate. The integrated circuitry can be incorporated into either or both of the two semiconductor chips. The two chips may be bonded together, using for example, flip-chip or conductive adhesive technology.
In many cases, laser diodes are preferred over LEDs as light sources. The laser diode, for example, provides a higher intensity beam than the LED. Additionally, its optical output also has a narrower wavelength spectrum, which is consequently less affected by dispersion caused by transmission through the optical fiber. “Laser diode” is a general term that includes two broad types of lasers. The first type of laser diode is an edge-emitting laser that emits light through an edge of an active region that comprises, for example, a p-n junction layer. The second type of laser diode is a vertical cavity surface-emitting laser (VCSEL).
Simply put, a VCSEL is a laser made of many layers, e.g., 600, which emits light vertically from a lower surface and in a direction parallel to the direction of its optical cavity, as opposed to an edge-emitting type laser structure. VCSELs have advantages over edge-emitting type structures because, for example, the edge-emitting type lasers must be precisely broken or cleaved individually to form each device during manufacturing. However, with VCSELs, literally millions of laser devices can be made simultaneously in an etching process.
A typical VCSEL comprises a plurality of layers of semiconductor material stacked on top of each other. A region centrally located within the stack corresponds to the active region comprising a p-n junction formed by adjacent p- and n-doped semiconductor layers. This active region is conventionally interposed between two distributed Bragg reflectors (DBRs), each DBR comprising a plurality of semiconductor layers with thicknesses selected so as to facilitate Bragg reflection, as is well-known in the art.
The term “vertical” in Vertical Cavity Surface Emitting Laser pertains to the fact that the planar layers comprising the DBRs and the active region, when oriented horizontally, are such that a normal to the planes faces the vertical direction and light from the VCSEL is emitted in that vertical direction, in contrast with horizontal emission emanating from a side of an edge-emitting laser. VCSELs offer several advantages over edge-emitting lasers, for example, VCSELs are typically much smaller than edge-emitting lasers. Furthermore, VCSELs produce a high intensity output. This latter advantage, however, can be negated if the emitted beam cannot be effectively captured and transmitted to an external location, e.g., via a waveguide. Typically, an optical coupling element such as a lens must be positioned adjacent to and aligned precisely with the VCSEL in order to achieve efficient optical coupling. This process reduces the cost effectiveness of using VCSELs in many instances, especially when a plurality of VCSELs is arranged in a one- or two-dimensional array.
Another advantage afforded by the VCSEL is increased beam control, which is provided by an aperture that is formed in one or more of the semiconductor layers. This aperture is conventionally formed by exposing the stack of semiconductor layers to water vapor to oxidize one of the layers. Initially outer edges of this semiconductor layer begin oxidizing. However, this oxidation progresses inwardly until the water vapor can no longer permeate the layer from the sides, wherein oxidation stops. Thus, a central region of the semiconductor layer remains un-oxidized. When the VCSEL is activated, current will flow through this central region and not the through the surrounding oxide barrier. In this manner, the current flow is confined to a small portion of the active layer. Recombination of electrons and holes within this region causes light to be generated only within a small, localized area within the VCSEL. For the foregoing reasons, this aperture and the layer containing it are conventionally referred to in the art as a current confinement layer.
VCSELs have a range of uses. For example, a specially designed VCSEL has been used to create an optical latch or optical state memory, the VCSEL transitioning and latching in the ON state when an optical input is received. Arrays of such VCSEL's open up possibilities for various massively parallel optical computing applications such as pattern recognition. VCSELs have data communications applications as well as would be clear to one skilled in the art, for example, as transmitters in parallel optical links. For more information about VCSELs, see, for example, “LASERS, Harnessing the Atom's Light,” Harbison et al., Scientific American Library, 1998, pages 169–177.
VCSEL arrays are commonly manufactured in a common cathode configuration, i.e., with all the laser cathodes connected together. The VCSEL does not begin lasing until the current through it exceeds a certain laser threshold value. The slope of a curve above this laser threshold is commonly referred to in the art as the differential quantum efficiency (DQE) of the VCSEL.
Vertical cavity surface emitting lasers (VCSELs) include first and second distributed Bragg reflectors (DBRS) formed on opposite sides of an active area. The VCSEL can be driven or pumped electrically by forcing current through the active area or optically by supplying light of a desired frequency to the active area. Typically, DBRs or mirror stacks are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. In conventional VCSELs, conventional material systems perform adequately.
However, new products are being developed requiring VCSELs which emit light having long-wavelengths. VCSELs emitting light having long-wavelengths are of great interest in the optical telecommunications industry. This long-wavelength light can be generated by using a VCSEL having an InP based active region. When an InP based active region is used, however, the DBRs or mirror stacks lattice matched to the supporting substrate and the active region do not provide enough reflectivity for the VCSELs to operate because of the insignificant difference in the refractive indices between the two DBR constituents.